Structural investigation of La9.33Si6O26- and La9AESi6O26+δ-doped apatites-type lanthanum silicate (AE=Ba, Sr and Ca) by neutron powder diffraction

Crystalline structures of La9.33Si6O26 and La9AESi6O26+δ (AE=Ba, Sr and Ca) doped apatites-type lanthanum silicates were investigated by X-ray and neutron powder diffraction at room temperature. The results obtained after different models testing show that the apatite structure is best described using the P63 space group. The loss of the mirror symmetry perpendicular to the ionic conduction channel direction results from heterogeneous La3+/AE2+ distribution of the sites so-called “4f”. The Rietveld refinements do not show splitting of the conduction oxygen image site but rather a very large spread of the nuclear density associated to this site. This effect is more pronounced for the La9AESi6O26+δ-doped compounds. Large anisotropic thermal displacement parameters are also observed for the oxygens associated to the isolated [SiO4], suggesting a rotation of this tetrahedron around the Si site. Lastly, vacancies were also systematically observed in the lanthanum nine-coordinated sites.